Wireless communications systems security Alexey Fomin, SUAI fomin@vu.spb.ru
Agenda Current security solutions in wireless systems (802.11) Open problems 2
Security Tasks Message authentication & privacy Node\Base Station mutual Authentication Key Management 3
Message Protection Message Protection Encryption (E(*)) (provides Privacy) Packet Number (PN) (provides Freshness) Message Integrity Code (MIC) (provides Authentication) Key hierarchy PMK = pair-wise master key PTK = pair-wise transient key (unicast traffic protection) GTK = group temporal key (broadcast traffic protection) 4
Key Management PMK Administrator provides STA and AP with PSK Only STA and AP are involved Administrator provides STA with key for AS (authentication server) EAP-TLS, EAP-PSK, Obtaining PMK in a distributed way by using AS (802.1X) 1. Request to AS 2. PMK STA AP 3. PMK AS 5
Obtaining PTK and GTK BSS mode PTK GTK 4-Way Handshake STA 1 STA 2 IBSS mode 4-Way Handshake GTK 6
Open Problems Authentication of broadcast frames Authentication of management frames Key management in ad-hoc (IBSS) networks 7
Broadcast Frames Authentication AP distributes GTK to all STAs during association process Broadcast data frames are protected by using of GTK Nodes can not identify real source of the broadcast frame Any STA could send broadcast data frames acting as AP 8
Possible Solutions Public Key Signature Computational overhead is too big MICs on pair-wise keys Network overhead is too big One/multi time signature Network overhead is too big TESLA Delayed authentication 9
TESLA Generation Usage K 0 =h(k 1 ) K n-2 =h(k n-1 ) K 3 K 1 K 2 K n-1 =h(k n ) K n K j-1 =H(K j ) K 0 verification key Broadcast source sends t for all messages during period j M M j K j 1 MIC(K j, j K j 1 M) Broadcast destinations Cache all the frames received during period j Verify that K j 2 =H(K j-1 ) Use K j 1 to validate the MICs of all frames received during the previous period j 1 Disadvantages: Delayed authentication, Time synchronization 10
Management Frames Mgmt frames are more important for functioning of the network Stronger protection should be used Broadcast Protection Issues Beacon Protection Deauthentication/deassociation Protection 11
Beacon Protection Problem Statement Beacon should be received by STA before authentication/association, i.e. before keys are available Possible solution Accept several initial beacons without authentication After authentication/association use TESLA 12
Beacon Protection Beacon is authenticated using TESLA TESLA requires time synchronization Time synchronization is provided in beacon Chicken and Egg problem 13
Deauthentication Protection DoS attack Disassociation or Deauthentication broadcast Associated Group (same GTK) Need to protect management frames that a Access Point uses to disassociate or deauthenticate. Otherwise attacker can forge such frames which results in a DoS attack. 14
Possible Solution AP distributes the commitment value CV = hash(cgtk) to STAs When AP sends broadcast Disassociation/Deauthenticate, it disclosures CGTK When an STA receives a protected broadcast, it accepts frame only if CV = hash(cgtk) This broadcast frame was successfully protected, because it is sent only once per session. One-time signature 15
Public Key Infrastructure (PKI) Binding between PK and user User certificate (CERT) CERT=<Name, Tstart, Tend, PK, CA Signature> Certificate management Issuing, renewal, revocation Done by trusted third party Certificate Authority (CA) 16
Key Management Centralized certification authority (CA), which provides STAs with certificates, exists in scenarios discussed previously There are scenarios (e.g. MANET), where such authority is not available (no fixed infrastructure) To use certificate in self-organized networks we need to distribute the functionality of CA Distribute signing procedure 17
Distributed Certification Authority Distributed signing procedure => distributed secret key Secret Sharing (t,n)-scheme SK CA is shared among all n nodes t<n nodes can make calculation with SK CA Less than t nodes can not 18
Thank You 19